Tuesday, August 19, 2008

Polygamy may be a key to a long life

Want to live a little longer? Get a second wife. New research suggests that men from polygamous cultures outlive those from monogamous ones.

After accounting for socioeconomic differences, men aged over 60 from 140 countries that practice polygamy to varying degrees lived on average 12% longer than men from 49 mostly monogamous nations, says Virpi Lummaa, an ecologist at the University of Sheffield, UK.

Rather than a call to polygamy, the research might solve a long-standing puzzle in human biology: Why do men live so long?

This question only makes sense after asking the same for women, who - unlike nearly all other animals - live long past the menopause.

One answer seems to be a phenomenon called the grandmother effect. For every 10 years a woman survives past the menopause, she gains two additional grandchildren, Lummaa says. It seems that doting on and spoiling grandchildren aids their survival, as well as furthering some of their grandmother's genes.

Men, by contrast, can reproduce well into their 60s and even 70s and 80s, and most researchers assumed this explained their longevity. But Lummaa and colleague Andy Russell wondered whether other factors explained the long lifespan of men, such as a grandfather effect.

But ultimately remarried men "don't end up with any more grandchildren," Lummaa says. "If anything the presence of a grandfather was associated with decreased survival of grandchildren."

Perhaps, Lummaa adds, the children of the first mother lose out on food and resources that go to the second mother's kids. "It's kind of the Cinderella effect."

Even fathers with only one wife provided no benefit to their grandchildren, a finding supported by previous research.

Biological selection

With the grandfather effect ruled out, Lummaa and Russell next wondered whether the constraints of human physiology explain male longevity. In the same way that men have nipples that evolved for women to nourish their young, male longevity might be a consequence of biological selection for long-lived women.

To answer this question, the researchers compared the lifespan of men from polygamous countries with those from monogamous nations.

If female survival is the main explanation for male longevity, then monogamous and polygamous men would live for about the same length of time. Instead, it seems that fathering more kids with more wives leads to increased male longevity. Men, then, live long because they're fertile well into their grey years.

The explanation could be both social and genetic. Men who continue fathering kids into their 60s and 70s could take better care for their bodies because they have mouths to feed. But evolutionary forces acting over thousands of years could also select for longer-lived men in polygamous cultures.

"It's a valid hypothesis and good prediction," says Chris Wilson, an evolutionary anthropologist at Cornell University in Ithaca, New York, who attended the talk. But the care and attention of several wives who depend on the social status of their ageing husband could explain everything. "It doesn't surprise me that men in those societies live longer than men in monogamous societies, where they become widowed and have nobody to care for them."